The present invention relates generally to a solid state optical device capable of actively modifying the intensity distribution across the entrance aperture of an electro-optic sensor to reduce saturation of the entire image by a bright point source. The invention may preferably be used in conjunction with a spatial light modulator (which can selectively block a region in an image) placed at an intermediate image plane or with a detector array capable of handling large over saturation without affecting the performance of the surrounding detector elements.
A problem with existing electro-optical systems is that the entrance aperture for collecting light from the scene has a uniform intensity profile. That is, within the entrance aperture transmission into the system is a fixed percentage, t, and outside the aperture the transmission is zero. The edge of the entrance aperture, where transmission goes from zero to t, creates diffraction within the electro-optic system Under normal operating conditions, where the intensity of light from the scene is within an order of magnitude or two, diffraction effects within the system do not cause problems, However, if a brightpoint source, of greater intensity than about three orders of magnitude above detector saturation, is within the system field of view, diffraction of the bright source by the edges of the entrance aperture will cause saturation of the surrounding regions of the image.
The invention substantially reduces effects of a bright source on an electro-optic system having conventional optical switching devices by providing an optical switch comprising a layer of optical switching material with optical thickness which is a preselected function of switch radius. The invention may be switched between a normal highly transparent window and soft aperture with graded attenuation. In the unswitched (OFF) state for the switching material, the switch is a high transmission (or reflection) window (or mirror). In the switched (ON) state, attenuation of transmission (or reflection) varies smoothly radially across the switch width, from substantially zero at the center of the switch to substantially 100% at the outer edge. The thickness profile may be selected according to an attenuation profile required for a specific system.
An optical switch structured according to the invention, being achromatic, protects against all types of high intensity sources. In operation, the switch modifies the transmittance function incident on the entrance aperture of a protected optical system. With the invention in place, a system may operate normally without sacrificing operational throughput. The invention can be retrofit to existing systems.
It is therefore a principal object of the invention to provide an optical switching device.
It is a further object of the invention to provide a spatially graded optical switch.
These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds.